This invention relates to combustible fuel ignition systems and more particularly to a fuel ignition system of the standing pilot type for cyclically verifying the operating integrity of the main fuel valve circuit and for rapidly de-energizing the main fuel valve upon loss of the pilot flame.
Modern fuel ignition systems frequently employ pilot ignition circuits wherein a pilot flame is established only intermittently in response to each demand for heat and is extinguished coincidently with the shutdown of the main burner. In an effort to enhance the reliability of such systems, it has been found desirable to repetitively verify the integrity of that portion of the apparatus circuitry which controls the operation of the main fuel valve. A system for verifying the integrity of such circuitry in a pilot ignition type system is shown and described in U.S. Pat. No. 4,131,412. There, a normally open contact set R1B of a pilot relay R1 is coupled in parallel with a normally closed contact set R2A of a flame sensing relay R2. Additionally a normally open contact set R2C of the relay R2 is coupled in series with the main valve relay R3, of which normally open contacts R3C actuate the coil 19 of the main fuel valve 14. At the closure of thermostat contacts THS and upon a failure in the flame sensing circuit 20 or in relay R2, relay R3 or, in the alternative, relay R1 is prevented from being energized depending upon the position of R2 at failure. Such a system inherently provides integrity verification since the relays are forced through an operating cycle with each successive re-ignition of the pilot flame. Many devices of this type employ a thermocouple to sense the presence or absence of a pilot flame. However, the time required for a thermocouple to sufficiently cool, thus signalling the loss of the pilot flame, may be undesirably long and cooling times on the order of 90 seconds are not unusual.
While such approaches have been found satisfactory in pilot ignition type systems, they are particularly unsuited for use in systems of the standing pilot type. This is so since a standing pilot ignition system includes a constantly-burning pilot flame and no opportunity is presented for repetitively verifying the integrity of that portion of the apparatus circuitry which controls the operation of the main fuel valve. Additionally, such approaches are unsuited for rapidly de-energizing the main fuel valve upon loss of the pilot flame in a standing pilot system.
An apparatus and method adapted for use in a standing pilot system to repetitively verify the operating integrity of the device used to actuate the main fuel valve and to rapidly de-energize the main fuel valve in the event of the loss of the pilot flame would be a significant improvement over the prior art.